It is well-known that many pneumatic devices, such as certain pneumatic tools, cylinders and control valves require the compressed air to contain a lubricant in the form of an oil fog. For that purpose, compressed air lines frequently include a lubricator unit that introduces a fog of oil into the compressed air as it flows through it. The lubricator includes an adjustable oil metering device so that the user can adjust the amount of oil introduced into the compressed air depending on the lubrication requirements of the pneumatic devices in question. The oil is automatically introduced into the compressed air, via the metering device, by virtue of a small pressure drop, typically of the order of 100 mbar, established in the lubricator by the flow sensor which is located in the lubricator's flow passage and which acts as a venturi type of device.
One requisite characteristic of the flow sensor is that, as the compressed air flow rate through the lubricator increases consequent on an increased demand for compressed air, the pressure drop increases approximately proportionally whereby the amount of oil introduced into the compressed air increases, also approximately proportionally. In other words, at a given setting of the metering device, the flow sensor ensures that the oil-to-air concentration remains substantially constant over a wide range of air flow rates through the lubricator. To that end, the flow sensor conventionally comprises a disc-like structure fixedly secured in the lubricator's flow passage, the air passing through an annular gap defined between the periphery of the disc and the internal surface of the flow passage. As the compressed air flow rate through the lubricator increases, the disc progressively deforms in a downstream direction whereby the cross-sectional area of the annular gap increases. In one typical design, the flow sensor comprises a flexible, plain-surfaced elastomeric disc secured in place at its centre which tends towards a conical configuration as the air flow rate through the lubricator increases. Accordingly, the circumference of the disc attempts to reduce, ie the rubber attempts to become circumferentially compressed, but the inherent nature of the material resists this. In practice, therefore, we have found that the periphery of the disc actually hinges so as to form four small `flaps`. In any event, because of the disc's inherent resistance to being deformed, the aforementioned design places a restriction on the maximum compressed air flow rate through the lubricator.
In an alternative, known design, the flow sensor comprises a diametric support rib to which a pair of semi-circular flaps are hinged by thin membranes. Again, this design of flow sensor is usually made of an elastomer as an integral moulding. Whilst, in that design, deformation of the sensor occurs more readily (by virtue of a hinging action of the flaps), the support rib occupies a relatively large proportion of the cross-sectional area of the lubricator's flow passage thereby limiting the flow capacity therethrough.
It is an object of the present invention to provide an improved flow sensor for a compressed air lubricator which, other things being equal, will permit of higher air flow rates.